@Bart (and others)
Here's my 2 cents on HP UPD driver issues;
Try deleting the HP .BUD files contained in the spool folder on the clients that are having these issues.
The .BUD files contain compiled driver configuration settings and could be the cause of your issues. These .BUD files are created everytime you load a printer or open the printer properties of and HP printer using the HP UPD drivers. In some strange way
these files tend to get 'corrupted' and cause the system to take a huge amount of time to load the settings.
I've seen this behaviour when trying to switch, between or to, HP printers in Office applications or trying to open the printer properties from the client computer.